Method for enhanced recovery of oil from oil reservoirs

ABSTRACT

The present invention provides a method for recovering oil from a subterranean reservoir using waterflooding, wherein the flooding fluid used in the waterflooding process comprises water and a pyruvate-rich xanthan gum having at least 5 weight % pyruvate. The use of a pyruvate-rich xanthan gum is expected to increase the recovery of oil by improving both the oil/water mobility ratio and the sweep efficiency in reservoirs with a high degree of heterogeneity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the U.S. Provisional ApplicationNo. 61/738,549, filed on Dec. 18, 2012.

FIELD OF THE INVENTION

The present invention relates to a process for recovering crude oil fromoil reservoirs using a flooding fluid comprising water and apyruvate-rich xanthan gum.

BACKGROUND OF THE INVENTION

In the recovery of oil from oil-bearing reservoirs, it is typicallypossible to recover only minor portions of the original oil in place byprimary recovery methods which utilize only the natural forces presentin the reservoir. Thus a variety of supplemental techniques have beendeveloped and used to increase oil recovery. A commonly used secondarytechnique is waterflooding which involves injection of water into theoil reservoir. As the water moves through the reservoir, it displacesoil therein to one or more production wells through which the oil isrecovered. One problem encountered with waterflooding operations is therelatively poor sweep efficiency of the water, i.e., the water canchannel through certain portions of the reservoir as it travels from theinjection well(s) to the production well(s), thereby bypassing otherportions of the reservoir. Poor sweep efficiency may be due, forexample, to differences in the mobility of the water versus that of theoil, and permeability variations within the reservoir which encourageflow through some portions of the reservoir and not others.

Various enhanced oil recovery techniques have been used to improve sweepefficiency. One way is to reduce the differences in the mobility of thewater versus the oil by thickening the water with a water solublepolymer. Polyacrylamides or partially hydrolyzed polyacrylamides havebeen used for oil recovery by thickening the water and reducing themobility of the water versus the oil. Polyacrylamides do not work wellin highly saline conditions, such as greater than 10 parts per thousandtotal dissolved solids (TDS) because they lose their ability toviscosify highly saline solutions. Polysaccharides have also been used(Larry W. Lake, Enhanced Oil Recovery, Society of Petroleum Engineers,2010, pages 314-353).

Another such technique involves increasing the viscosity of the waterusing non-biodegradable thickening agents such as polyvinyl aromaticsulfonates as described in U.S. Pat. No. 3,085,063.

Surfactants have been also used in aqueous media for enhanced oilrecovery. Surfactants have been found to effectively lower theinterfacial tension between oil and water and enable mobilization oftrapped oil through the reservoir. For example, U.S. Pat. No. 8,163,678describes methods for enhanced oil recovery using a surfactantformulation comprising (a) an alkylaromatic sulfonate; (b) an isomerizedolefin sulfonate (c) a solvent; (d) a passivator; and (e) a polymer.Polymers disclosed therein include xanthan gum, partially hydrolyzedpolyacrylamides (HPAM) and copolymers of 2-acrylamido-2-methylpropanesulfonic acid and/or sodium salt and polyacrylamide (PAM) commonlyreferred to as AMPS copolymer. Molecular weights (M_(w)) of the polymersrange from about 10,000 daltons to about 20,000,000 daltons. Polymersare used in the range of about 500 to about 2500 ppm concentration, inorder to match or exceed the reservoir oil viscosity under the reservoirconditions of temperature and pressure.

In addition to demonstrating good viscosifying at low shear and lowconcentration, several other attributes are desirable for viscosityenhancers to be useful in oil recovery. For off-shore drillingoperations, sea water is used for waterflooding, so good solubility ofthe viscosity enhancer in cold, highly saline water is very desirable.The salinity of the water in subterranean hydrocarbon reservoirs mayalso vary a great deal. For example, the Minas oil field in Indonesiahas total dissolved solids of between 0.2 and 0.3 weight percent. Otherreservoirs may have salinities as high as or higher than 2.0 percentsodium chloride and over 0.5 percent calcium chloride and magnesiumchloride. Still other reservoirs can have total dissolved solids inexcess of 6 weight percent and in some cases in excess of 20 weightpercent. For these high salt brines, the divalent ion concentrationsincluding calcium and magnesium, can well be in excess of 0.1 weight %.Salinity and the presence of divalent ions including calcium andmagnesium can have an effect on the phase behavior of the variouschemicals used in oil recovery.

It is also desirable that the viscosity enhancer is thermally stable,because the oil field deposit may be geothermally heated. Mostreservoirs are warm but cool down near the injector well under prolongedwater flood. Near the injector well bore, the temperatures are near 25°C. As the water moves away from the injector to the production well,temperatures rise and the amount of oil that is likely to be left in theformation also increases as distance away from the injector to theproduction well increases. Hence it is desirable to have a water solublepolymer that increases the solution viscosity at low shear and at a hightemperature, e.g., 80° C.

It is also desirable that the water/brine soluble polymer used be stablefor the long periods needed for the polymer solutions to transit the oilreservoir from the injector well to the producer well. Hence it isdesirable to use a polymer that is stable against hydrolytic, thermaland anaerobic (no air present) biological degradation. It is alsodesirable that the polymer solution not degrade under the high shearconditions encountered by the solution as it is pumped by a highpressure pump or at high shear zones in throttle valves used to regulatewater flows into the injector wells. It is important that the polymer insolution is not retained in the rock matrix in the oil reservoir. Thatis, only a small amount of polymer can be removed from the solution byadsorption or retention on or in the rock matrix. Otherwise thetreatment becomes ineffective if the polymer is removed from the brinesolution. Finally, biodegradability in air or aerobic conditions and/orsourcing from renewable resources are also desirable especially when thepolymer solution that is eventually coproduced with the oil is disposedof at sea or in waste water treatment facilities. There is therefore aneed for a method to improve sweep efficiency of waterfloodingoperations using cost-effective, biodegradable materials that exhibitshear-thinning properties and thus exhibit lower viscosity duringinjection and increased viscosity in the oil reservoir. U.S. PatentApplication Publication 2012/0021112 discloses certain xanthan gums withimproved characteristics. In particular, the xanthan gums are obtainedfrom Xanthomonas campestris strains, pathovar cynarae CFBP 19, juglandisCFBP 176, pelargonii CFBP 64, phaseoli CFBP 412 or ATCC 17915,celebenois ATCC 19046, or corylina CFBP 1847 or from a derivative orprogeny thereof.

SUMMARY OF THE INVENTION

This invention relates to the recovery of oil from a subterraneanreservoir using waterflooding. In one aspect, this invention provides amethod for recovering oil from a reservoir by water flooding,comprising:

-   -   (a) introducing an aqueous flooding fluid into the reservoir,        wherein at least one portion of said flooding fluid comprises a        xanthan gum characterized as having a pyruvic acid content of at        least 5.0 weight % (w/w);    -   (b) displacing oil in the reservoir with said flooding fluid        into one or more production wells, whereby the oil is        recoverable.

In another aspect, this invention is an aqueous flooding fluid forenhanced oil recovery, wherein at least one portion of said floodingfluid comprises water and 0.007 to 3 weight % a xanthan gumcharacterized by pyruvic acid content of at least 5.0 weight %.

In another aspect, the present invention provides a method of making anaqueous flooding fluid for use in waterflooding, comprising:

-   -   (a) providing a xanthan gum characterized by pyruvic acid        content of at least 5.0% (w/w); and    -   (b) adding the xanthan gum to at least one portion of water used        in waterflooding.

In another aspect, the invention provides a method for recovering oilfrom a reservoir by waterflooding, comprising:

-   -   (a) introducing an aqueous flooding fluid into the reservoir,        wherein at least one portion of the flooding fluid comprises        0.007 to 3 weight % of a xanthan gum characterized by pyruvic        acid content of at least 5.0 weight % (w/w);    -   (b) displacing oil in the reservoir with said flooding fluid        into one or more production wells, whereby the oil is        recoverable.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows plots of the effect of pyruvate content on the viscositytemperature profile of 0.5 weight % xanthan gum in 0.1% sodium chloridesolutions.

FIG. 2 shows plots of the midpoint transition temperature as a functionof salt concentration for 0.5 weight % xanthan gum solutions.

FIG. 3 shows plots on the effect of thermal degradation on apparentviscosity of xanthan gum solutions at low shear of 1 sec⁻¹ in saltwater.

DETAILED DESCRIPTION

The following definitions are provided for the special terms andabbreviations used in this application:

The term “pyruvate-rich xanthan” as used herein refers to a xanthan gumcharacterized by a pyruvic acid content of at least 5.0% (w/w), in whichthe pyruvic acid is incorporated into the xanthan as pyruvate estermoieties. The term “gum” refers to a non-starch, non-pectin carbohydratepolymer derived from land or sea plants, or microorganisms.

The term “water” refers to water that can be supplied from any suitablesource, and can include, for example, sea water, brine, productionwater, water recovered from an underground aquifer, including thoseaquifers in contact with the oil, or surface water from a stream, river,pond or lake. As is known in the art, it may be necessary to removeparticulates from the water prior to injection into the one or morewells. Water also includes various “synthetic” brines or sea waterhaving defined total dissolved solids.

As used herein, the abbreviation “ppt” stands for parts per thousand.“Total dissolved solids” (TDS) refers to the amount of inorganicmaterial in the water as described herein, and does not include anydissolved polymer material that may be present in the water.

The term “mobility” is defined as the ratio of the relative permeabilityof the fluid to its viscosity at reservoir conditions. The relativepermeability for water is evaluated at the average water saturation ofthe swept zones of the oil reservoir, typically at residual oilsaturation. Here water saturation refers to the fraction of the voidvolume occupied by water. The relative permeability of the oil isevaluated at the oil saturation of the unswept zone in the oilreservoir, typically at residual water saturation (Boatright, K E, 2002,Basic Petroleum Engineering Practices, 9.6; see also IntegratedPetroleum Management—A Team Approach, (A. Sattar and G. Thakurm,PennWell Books, Tulsa, Okla., 1994)).

As used herein, “shear thinning” refers to the reduction of viscosity ofa liquid (such as that portion of the flooding fluid comprising thepyruvate-rich xanthan) under shear stress. “Viscosity” refers to theresistance of a liquid such as water or oil to flow. The term “sheardependent viscosity ratio” as used herein is defined as the ratio of thesolution viscosity measured at a given temperature and at a shear rateof 1 sec⁻¹ to the solution viscosity measured at that same temperatureand at a shear rate of 10 sec⁻¹. The term “temperature dependentviscosity ratio” as used herein is defined as the ratio of viscosity ofidentical aqueous solutions of xanthans at two different temperatures,such as at 20° C. and 60° C. When discussing viscosity of compositionscomprising xanthan gums, these parameters may be generally determined byuse of the methods and apparatus specifically referred to in theexamples or similar methods and apparatus.

As used herein, a polymeric material is considered “soluble” in a liquidif the “solution” can be passed through a 0.2 micron filter withoutsubstantial pressure buildup. One skilled in the art can appreciate thatno characterization is made regarding the interaction of the individualpolymer molecules with the liquid molecules in such solutions.Filterability of the solution to assess dissolution of the polymer canbe determined using a method described by the American PetroleumInstitute (API: “Recommended Practices for Evaluation of Polymers Usedin Enhanced Oil Recovery Operations” RP63, First Edition, 1990, page33).

The term “production well(s)” refers to well(s) through which oil andwater are withdrawn from a reservoir. An oil reservoir or oil formationis a subsurface body of rock having sufficient porosity and permeabilityto store and transmit oil. The terms “injection well(s)” and “injector”refer to well(s) that are used to pump water or water mixtures into anoil reservoir for waterflooding purposes.

The invention relates to the recovery of oil from a subterraneanreservoir using waterflooding. Waterflooding is a technique that iscommonly used for secondary oil recovery from oil reservoirs. In thistechnique, water is injected through one or more wells into thereservoir, and as the water moves through the reservoir, it acts todisplace oil therein to one or more production wells through which theoil is recovered.

According to this invention, the efficacy of waterflooding is improvedthrough the use of a pyruvate-rich xanthan. We have found thatpyruvate-rich xanthan is readily soluble in cold saline water andmaintains the ability to provide a high viscosity at low shear in highlysaline solutions. This is particularly apparent compared topolyacrylamide polymers, which do not maintain their viscosifying effectin salty solutions, such as greater than 10 parts per thousand TDS. Useof pyruvate-rich xanthan would be an advantage for operations on an oilplatform at sea (for example, the North Sea), since the water used forflooding the oil reservoir is cold sea water and there are no or limitedresources on typical platforms to heat sea water to help dissolve thepolymer. Also, the pyruvate-rich xanthan does not thermally degrade atmodest temperatures compared to available xanthans with lower pyruvatecontent, which lose their viscosifying effect after extended treatmentat elevated temperatures. Thus, in one aspect, the present inventionprovides a flooding fluid for use in waterflooding operations comprisingwater and a pyruvate-rich xanthan.

The invention also relates to a method for recovering oil from areservoir by waterflooding, through introducing an aqueous floodingfluid into the reservoir. In one aspect, the flooding fluid comprises apyruvate-rich xanthan.The pyruvate-rich xanthan may be prepared according to methods describedin U.S. Patent Application Publication 2012/0021112, the entiredisclosure of which is incorporated by reference herein.

The pyruvate-rich xanthan can be prepared using a Xanthomonas campestrisstrain. As used herein, Xanthomonas campestris strain (or X. campestrisstrain) is a strain of the bacterial species which causes a variety ofplant diseases. A “derivative” of a X. campestris strain is in thecurrent context a bacterial cell derived from X. campestris, that haspreserved substantially all genomic information of the X. campestrisstrain—the derivative may, however, differ from the parent X. campestrisstrain by having recombinantly introduced genetic modifications (e.g. inthe genome or in the form of a plasmid), which do not adversely affectthe functionality of the xanthan gum gene cluster. A “progeny” of a X.campestris strain is a bacterial cell that is obtained by culture of aX. campestris strain—hence, the progeny may include later generationbacterial cells which are not genetically identical with the original X.campestris strain (due, e.g., to the emergence of natural recombinationevents, or spontaneous mutations), but which do not include any geneticchanges which adversely affect the functionality of the xanthan gum genecluster. A X. campestris strain producing xanthan gum having a highpyruvic acid content is a strain which produces Xanthan gum having apyruvic acid content of at least 5% (w/w), preferably at least 5.3%,more preferably at least 5.5%, such as 5.5 to 7%.

The pyruvic acid to acetic acid weight/weight ratio in the pyruvate-richxanthans may be least 0.5, preferably at least 0.6, at least 0.7, atleast 0.8, at least 0.9, or at least 1.0, up to about 2. Particularlypreferred is a ratio of about 0.9 to about 1.3, such as about 1.1.

The pyruvate-rich xanthans may have solubility in mixed salt brine up tothe solubility limit of 350 parts per thousand (ppt) total dissolvedsolids (TDS), wherein solubility is measured by the ability of thesolution to pass through a 0.2 micron filter. The pyruvate-rich xanthansmay also have a viscosity measured at 0.1% xanthan in brine solutioncontaining salt composition up to 265 ppt TDS at shear rate of 1 sec⁻¹giving viscosity of at least 40 cP at 25° C. and at least 10 cp at 85°C. Additional details of the pyruvate-rich xanthan may be found in U.S.Patent Application Publication 2012/0021112, including viscosity ofcompositions comprising the pyruvate-rich xanthan in water with variedsalt content.

Solutions of xanthan gum undergo a conformational transition duringheating which is associated with the change from a rigid, ordered,generally helical state at low temperature to a more flexible,disordered random coil state at high temperatures. This conformationalchange was first observed as a sigmoidal change in viscosity (Jeanes,A., Pittsley, J. E. and Senti, F. R. “Polysaccharide B-1459: a newhydrocolloid polyelectrolyte produced from glucose from bacterialfermentation,” J. App. Polym. Sci., 1961, 5, 519-526). The temperatureof the conformational transition increases with increasing ionicstrength of the solution (Sworn, G. “Xanthan gum.” in Phillips, G. O.and Williams, P. A. (Eds.) Handbook of Hydrocolloids, WoodheadPublishing Ltd, Cambridge, 2000, 103-116). The temperature at which theconformational transition occurs has also been shown to be dependent onthe pyruvic acid content of the xanthan molecule and it has beenreported that the transition temperature decreases with increasingpyruvate content (Morrison, N. A., Clark, R., Talashek, T. and Yuan, C.R. “New forms of xanthan gum with enhanced properties” in Gums andStabilisers for the Food Industry 12, P. A. Williams and G. O. Phillips(eds.), RSC, Cambridge, 2004, 124-130 and Morris, E. R. “Molecularorigin of xanthan solution properties” in Sandford, P. A. and Laskin, A.(Eds) Extracellular microbial polysaccharides, ACS symposium series no.45, American Chemical Society, Washington, D C, 1977, 81-89).

As demonstrated herein, the pyruvate-rich xanthans exhibitconformational transition behavior that is particularly useful for usein a flooding fluid for enhanced oil recovery.

Pyruvate-rich xanthan gums, having at least 5 weight % pyruvate, providea viscosity, measured at 0.1% xanthan in brine solution containing asalt composition up to 265 ppt at shear rate of 1 sec⁻¹, of at least 40centipoise (cP) at 25° C. and at least 10 cP at 85° C. Alternatively,such pyruvate-rich xanthan gums provide a viscosity measured at 0.3%xanthan in 1 weight % NaCl solution at a shear rate of 0.01 s⁻¹ at 23±2°C. of at least 80 Pa·s (wherein 1 Pa·s=1000 centipoise). Mixtures ofxanthan gums may be used, provided that, in aggregate, the mixtures arecharacterized by the pyruvic acid content and viscosity describedherein.

Without being bound by theory, the combination of viscosity andviscosity at high temperature of the pyruvate-rich xanthans is dependenton the pyruvate content of the xanthan. Too low pyruvate may providesolubility as defined herein and sufficient viscosity at lowtemperature, but would not provide adequate viscosity at hightemperature.

The suitability of materials for flooding fluids may also be assessedfor the following factors:

1) Retention of the polymer in solution in the rock matrix in the oilreservoir as measured using a method described by Maria de Malo andElizabeth Lucas, “Characterization and Selection of Polymers for FutureResearch on Enhanced Oil Recovery”, Chemistry & Chemical Technology,Vol. 2, no. 4, 2008, pp 295-303. A suitable value for commercial use isless than 50 microgram retention of polymer per gram of rock. Thepyruvate-rich xanthan compositions described herein may exhibit lowpolymer retention in rock matrix.

2) Thermal degradation of the polymer solution under air free conditionsand at a temperature consistent with the reservoir conditions. To assessthis property, the polymer solution(s) may be held in an inert bottle(s)and sampled periodically to measure the viscosity for a range of shearfrom 1 to 10 sec⁻¹ and as a function of time at the temperature. Adesirable value for commercial use is less than 10% loss of viscosity(measured at shears of 1 and 10 sec⁻¹) over a 1 year period in syntheticsea water at 85° C. The pyruvate-rich xanthan compositions describedherein have minimal viscosity loss compared to previous oil-recoverypolymers when measured under these conditions.

3) Shear degradation of the polymer in solution using a method describedby the American Petroleum Institute (API: “Recommended Practices forEvaluation of Polymers Used in Enhanced Oil Recovery Operations” RP63,First Edition, 1990, page 60). For commercial use of a polymer, adesirable value in this test is retention of shear viscosity greaterthan 90%.

4) Anaerobic biodegradation of the polymer solution using enrichmentcultures inoculated with environmental samples or inoculated withmicrobes from culture collection that are known to degrade polymers.These enrichments should be done using different media to accentuatedifferent metabolisms. For example one medium would use sulfatereduction for metabolism (to address sulfide formation). In this case,the microbes use the polymer as the carbon source (or electron donor)while sulfate (a common anion in the injection water) is the electronacceptor. For commercial use of a polymer, a desirable value in thistest is less than 10% loss of viscosity over a 1 year period.

Accordingly, described herein is a flooding fluid useful forwaterflooding comprising water and a cold-water soluble pyruvate-richxanthan as described above. The aqueous flooding fluid for use inwaterflooding, comprising about 0.007% to about 3% weight of a xanthangum characterized by pyruvic acid content of at least 5.0%(weight/weight) in water.

The concentration may be in the range of about 0.05% to about 1%(weight/weight). The aqueous flooding fluid may comprise 0.05 to 0.2weight % of the xanthan gum and may also include greater than 10 partsper thousand total dissolved solids (TDS).

This invention provides an advantage to existing technology in thatflooding fluid comprising a pyruvate-rich xanthan as defined aboveexhibit shear-thinning properties such that the solution exhibits lowviscosity at high shear rates and increased viscosity at low shearrates.

The flooding fluid comprises water, wherein at least a portion of saidwater comprises a pyruvate-rich xanthan. The water may have salinity upto 10 ppt total dissolved solids (TDS), or greater than 10 ppt TDS, orgreater than 25 ppt TDS, greater than 35 ppt TDS and up to thesolubility limit of mixed salts in water which is about 350 ppt.Synthetic sea water as defined by API standards has a TDS of about 30 to40 ppt, such as 34 ppt. Typical oil reservoir brines may have from 10 to275 ppt TDS, such as 40 to 85 ppt TDS. The ratio of divalent ions tomonovalent ions may also have an impact on low shear viscosityperformance of dissolved polymers. The pyruvate-rich xanthan useful inthe invention exhibits particularly good rheological properties insolution. The viscosity values observed for the pyruvate-rich xanthanare superior to those of existing commercial products. The mainadvantage obtained from this property of the pyruvate-rich xanthan isthat the amount of xanthan gum in the flooding fluid may be reducedwhile retaining high viscosity compared to a flooding fluid containingprevious xanthan gums.

The pyruvate-rich xanthan also exhibits particularly good rheologicalproperties in highly saline solutions.

The pyruvate-rich xanthan may have a viscosity measured at 0.1% xanthanin synthetic sea water at a shear rate of 1 sec⁻¹, and a temperature of25±2° C. which is at least 40 cp. The viscosity measured at 0.1% xanthanin synthetic sea water at a shear rate of 1 sec⁻¹ and at a temperatureof 25±2° C. may be in the range from 40 to 130 cp, such as in the rangefrom about 45 to about 125 cp.

The pyruvate-rich xanthan may have a viscosity at a shear rate of 1sec⁻¹ in 265 ppt TDS synthetic brine (characterized in Table 6 below) ofat least 40 cp at 25° C. However, viscosity will decrease withincreasing temperature. For example at 85° C. at the same shear and samepolymer composition in the same synthetic brine it may be 12 cp.

Further, this invention provides a method of making an aqueous floodingfluid for use in waterflooding, comprising:

-   -   (a) providing a xanthan gum characterized by pyruvic acid        content of at least 5.0% (w/w) (as described above); and    -   (b) adding the xanthan gum to at least one portion of water used        in waterflooding.        The pyruvate-rich xanthan can be added as a solid powder to at        least one portion of the flooding fluid. The concentration of        the pyruvate-rich xanthan in at least one portion of the        flooding fluid can be in the range of about 0.007% to about 3%        (weight of the pyruvate-rich xanthan/total weight of the at        least one portion of flooding fluid comprising said        pyruvate-rich xanthan). For preparing relatively small amounts        of flooding fluid, the xanthan may be added in a batch process,        wherein a defined weight of xanthan is added to a defined volume        of water to form a solution. The xanthan solution may be        produced in any suitable vessel, such as a tank, vat, pail and        the like. Stirring or mixing is useful to provide effective        contact of the bulk xanthan powder with water. The xanthan gum        dissolves substantially completely in water at about 25° C.        within about 14 hours. Preferably the solution is produced in        about 1 hour or less, such as in about 30 minutes.

For larger scale operations such as waterflooding, the xanthan may beadded continuously to a stream of water. Due to the good solubility ofthe xanthan in water, it is contemplated that the process may proceedwithin a pipeline in which the components of the dispersion are chargedat one end of the pipeline and form the solution as they proceed downthe length of the pipeline. For example, the xanthan powder may be mixedby metering solid xanthan at a defined rate with water as it passesthrough a pipeline, with or without added mixing, such as through staticmixers. Alternatively, the xanthan may be mixed with a small portion ofwater to form a slurry and diluted to the final concentration by addingthe slurry to additional water as they pass through a pipeline, with orwithout added mixing, such as through static mixers.

This invention also relates to the recovery of oil from a subterraneanreservoir using waterflooding. In one aspect, the invention provides amethod for recovering oil from a reservoir by waterflooding, comprising:

-   -   (a) introducing an aqueous flooding fluid into the reservoir,        wherein at least one portion of the flooding fluid comprises        0.007 to 3 weight % of a pyruvate-rich xanthan gum characterized        by pyruvic acid content of at least 5.0 weight % (w/w); and    -   (b) displacing oil in the reservoir with said flooding fluid        into one or more production wells, whereby the oil is        recoverable.        Thus, in one aspect, the pyruvate-rich xanthan is added to a        volume of water and injected into the well(s), optionally        followed by the injection of additional water (not containing        the pyruvate-rich xanthan). The at least one portion of the        flooding fluid containing the pyruvate-rich xanthan exhibits low        viscosity during injection into the reservoir and higher        viscosity when flowing through the reservoir. This process can        be repeated one or more times if necessary. At the injection        well(s), which is under high pressure and high shear, the        relative viscosity of at least one portion of the flooding fluid        comprising the pyruvate-rich xanthan is low, whereas as at least        one portion of the flooding fluid flows into the reservoir, the        shear decreases and the relative viscosity increases. The        pyruvate-rich xanthan can also be added to the entire volume of        flooding fluid, as long as the backpressure at the injection        well(s) does not become too high. As is known to those skilled        in the art of oil recovery, the bottom well pressure of the        injector cannot exceed the strength of the rock formation,        otherwise formation damage will occur at a given flow rate.        Adjustments can be made by reducing the flow of the injection        water, adding water to decrease viscosity, or by adding water        mixed with a pyruvate-rich xanthan to increase viscosity in        order to improve the efficacy of oil recovery.

In one aspect, pyruvate-rich xanthan is added to the flooding fluid inorder to increase the viscosity of at least one portion of the water inthe flooding fluid, thereby improving the displacement of oil to theproduction well(s). To achieve optimal efficiency in waterfloodingoperations, it is desirable that the mobility of the water be less thanthe mobility of the oil. The oil mobility is calculated by the formulak_(o)/μ_(o), where k_(o) is the relative oil permeability measured atresidual water saturation and μ_(o) is the oil dynamic viscositymeasured at reservoir conditions. Similarly, the water mobility iscalculated by k_(w)/μ_(w), where k_(w) is the relative waterpermeability measured at residual oil saturation and μ_(w) is the waterdynamic viscosity measured at reservoir conditions. In typicalwaterflooding operations the water mobility is greater than the oilmobility, thus the water will tend to channel or finger through the oil.When the pyruvate-rich xanthan is added to the at least one portion ofthe flooding fluid as described herein, the addition of thepyruvate-rich xanthan increases the viscosity of the at least oneportion of the water, thereby reducing the effective water mobility.Thus, the oil is more likely to be driven towards the productionwell(s).

The viscosity of at least one portion of the flooding fluid comprisingthe pyruvate-rich xanthan is about 30% higher at low shear rates of 1sec⁻¹ or less than the viscosity of the same polymer in solutionmeasured at the same temperature but at a high shear rate of 10 sec⁻¹ orgreater. Consequently a figure of merit that will be used to illustratethe degree of shear thinning is the shear dependent viscosity ratiomeasured at a specific temperature at different shear rates,specifically at shear rates of 1 sec⁻¹ and 10 sec⁻¹. Using this figureof merit, in one aspect, this viscosity ratio for at least one portionof the flooding fluid comprising pyruvate-rich xanthan is at least 1.3,preferably at least 1.8, more preferably at least 2.0, or at least 2.5.In a stratified oil-bearing formation the permeability of differentgeological oil-bearing layers may differ, and as a result the injectedwater could initially reach the production well through the mostpermeable layer before a substantial amount of the oil from other, lesspermeable, layers is retrieved. This breakthrough of injection water isproblematic for oil recovery, as the water/oil ratio retrieved from theproduction well will increase and become more unfavorable during thelifetime of the oil field. The addition of the pyruvate-rich xanthan toat least one portion of the flooding fluid is expected to result in lesswater flooding the more permeable zones in a reservoir, thus reducingthe chance of fingering of flooding fluid through these more permeablezones of the oil bearing strata and improving sweep efficiency.Additional materials can optionally be added as thickening agents orsurface active agents to enhance the sweep efficiency of the floodingfluid and/or reduce water mobility. These materials include at least oneof the members of the group consisting of hay, sugar cane fibers, cottonseed hull, textile fibers, shredded paper, bentonite, rubber pulp, woodshavings and nut hulls, provided that these materials together with apyruvate-rich xanthan provide the desired viscosity, concentrationand/or particle size distribution.

In addition, the additional materials may include propanediolthickeners, such as one or more members of the group consisting of1,3-propanediol; an oligomer of 1,3-propanediol; a homopolymer of1,3-propanediol; and a heteropolymer of 1,3-propanediol, wherein saidheteropolymer is synthesized using at least one C₂ through C₁₂ comonomerdiol, as described in the commonly owned and copending U.S. applicationSer. No. 12/023,166. An “oligomer” of 1,3-propanediol has a degree ofpolymerization of 2-6, whereas a “polymer” has a degree ofpolymerization of at least 7. A “homopolymer” of 1,3-propanediol is apolymer synthesized using monomers of 1,3-propanediol. A “heteropolymer”of 1,3-propanediol is a polymer synthesized using 1,3-propanediolmonomers as well as one or more additional C₂ through C₁₂ straight-chainor branched comonomer diols. Additional thickeners includepolyacrylamide, carboxymethylcellulose, polysaccharide, polyvinylpyrrolidone, polyacrylic, and polystyrene sulfonates, and ethylene oxidepolymers, as described in U.S. Pat. No. 3,757,863; and methyl cellulose,starch, guar gum, gum tragacanth, sodium alginate, and gum arabic, asdescribed in U.S. Pat. No. 3,421,582. Each of the thickeners can be usedalone, or in combination with one or more other thickeners as describedabove. Surfactants, such as acid salts of amido-acids as described inU.S. Pat. No. 2,802,785 can also optionally be added. Surfactants andthickeners can also be used in combination.

Thus, in one aspect, the additional materials that are added to floodingfluids of the invention are preferably biodegradable, such as starch,guar gum, sodium alginate, gum arabic and methyl cellulose.

The pyruvate-rich xanthan is added to a volume of water and injectedinto the well(s), followed by the injection of additional water. Thisprocess can be repeated one or more times if necessary. At the injectionwell(s), which is under high pressure and high shear, the relativeviscosity of at least one portion of the flooding fluid comprising thepyruvate-rich xanthan is low, whereas as at least one portion of theflooding fluid flows into the reservoir, the shear decreases and therelative viscosity increases. The pyruvate-rich xanthan can also beadded to the entire volume of flooding fluid, as long as thebackpressure at the injection well(s) does not become too high. As isknown to those skilled in the art of oil recovery, the bottom wellpressure of the injector cannot exceed the strength of the rockformation, otherwise formation damage will occur at a given flow rate.Adjustments can be made by reducing the flow of the injection water,adding water to decrease viscosity, or by adding water mixed with theone or more pyruvate-rich xanthan to increase viscosity in order toimprove the efficacy of oil recovery.

The flooding fluid can be recovered as it exits the production well(s)and at least one portion of the recovered flooding fluid can be reused,i.e., reinjected into the reservoir. Prior to reinjection into thereservoir, additional pyruvate-rich xanthan as defined above can beadded to at least one portion of the recovered flooding fluid. Theadditional pyruvate-rich xanthan can be added at a concentration ofabout 0.007% to about 3% (weight of one or more pyruvate-richxanthan/weight of at least one portion of flooding fluid).Alternatively, at least one portion of the flooding fluid exiting theproduction well(s) can be disposed of, for example by disposal at sea,in a disposal well, or in a wastewater pond.

EXAMPLES

The present invention is further illustrated in the following Examples.It should be understood that these Examples are given by way ofillustration only and are not meant to be limited to said Examples. Fromthe above discussion and these Examples, one skilled in the art canascertain the essential characteristics of this invention.

Materials Used

Samples of xanthans of various pyruvate and acetate content assummarized in Table 1 were obtained. Some xanthans are commerciallyavailable from DuPont™ Danisco under the tradename GRINDSTED® or undervarious material numbers. High pyruvate xanthans have xanthate contentof at least 5%, such as 5.5 to 7 weight %. In some cases, xanthans withlower pyruvate content can be obtained by hydrolysis of pyruvate-richxanthans by adjustment of the xanthan gum broth to acidic pH and holdingat elevated temperatures prior to recovery of the gum by alcoholprecipitation. The reduction in pyruvate content is a function of the pHand time held at the elevated temperature. Other xanthans with lowerpyruvic acid content can also be obtained commercially as indicated inTable 1. Pyruvate and acetate content can be quantified by HPLC afterhydrolysis of the polymer.

TABLE 1 Weight % Sample Origin Pyruvate Acetate PRX-1 DuPont ™ Danisco 6.49 5.1 GRINDSTED ® Xanthan MAS-SH PRX-2 DuPont ™ Danisco A35300  5.5-7 4-6.5 PRX-3 Pilot scale fermentation 5.1 4.5 X-1 DuPont ™Danisco 3.7 6.2 GRINDSTED ® Xanthan 80 X-2 Pilot scale fermentation 2.34.4 X-3 Pilot scale fermentation 3.9 4.2 X-4 Pilot scale fermentation4.5 4.5 X-6 DuPont ™ Danisco A43300 1.5-3.4 4-6.5 X-7 DuPont ™ DaniscoA45100 3.5-4.5 4-6.5Additional non-xanthate materials include:SG-1: succinoglycan available from DuPont™ Danisco under the tradedesignation A11100.PA-1: polyacrylamide formerly available under the tradename Performa® PA9510 from Hercules Corporation.Samples of the various xanthans of Table 1 were dissolved in deionizedwater. Sodium chloride was added as powder and dissolved after hydrationof the xanthan gum to prepare solutions with defined salt content.Viscosity as a function of temperature was measured using a Rheometer(Anton Parr MCR 501) at a shear rate of 10 s⁻¹, heating rate of 2° C.min⁻¹, with a 50 mm diameter, 2° angle cone. The results were plottedand representative results for solutions of 0.5% xanthan gum in 0.1%NaCl are shown in FIG. 1.As discussed above, xanthans exhibit a conformational transition from anordered helical structure at low temperature to a disordered random coilconfiguration at higher temperature. As a result, the viscosity ofxanthan solutions decreases significantly when the xanthan is in thedisordered state. The conformational transition can be observed wherethe slope of the temperature dependent line is the steepest.The temperature dependent viscosity ratio (the ratio of the viscosity at60° C. to the viscosity at 20° C.) of the samples is summarized in Table2.

TABLE 2 X-2 X-3 X-4 PRX-3 Viscosity ratio 0.65 0.78 0.83 0.95The data in Table 2 demonstrate that the temperature dependent viscosityratio increases with increasing pyruvate content. Prior to the onset ofthe conformational transition, the viscosity of pyruvate-rich xanthanPRX-1, with pyruvate content over 5 weight %, is virtually constant. Thetemperature dependent viscosity ratio of pyruvate-rich xanthans isgreater than 0.85, preferably greater than 0.90 or 0.95. This effect isindependent of the salt concentration within the range studied. Incontrast, the viscosity of xanthans with less than 5 weight % pyruvatedecreases with increasing temperature, with temperature dependentviscosity ratios less than 0.85. Even when the salt content is increasedthe low-pyruvate xanthans still show a decrease in viscosity withincreasing temperature. In both cases the viscosity decreases by anorder of magnitude as a result of the conformational transition. Also,the viscosity above the transition temperature is significantly higherfor the pyruvate-rich xanthan compared to xanthans with less than 5weight % pyruvate. The temperature at the mid-point of theconformational transition for solutions of PRX-1 and X-1 at 0.5 weight %xanthan for was taken and plotted as a function of salt concentrationand the results are shown in Table 3 and FIG. 2. The temperature of theconformational transition increased with increasing ionic strength ofthe solution.

TABLE 3 Transition temperature (° C.) NaCl concentration (%) PRX-1 X-10.025 66 0.05 71 0.05 58.5 71 0.1 63 74.5 0.125 64.5 77 0.15 68 79.50.175 72 82.5 0.2 74.5 84.5 0.225 77 86.5 0.25 78 88 0.275 80.5 89.5 0.382 0.325 83.5 0.35 85 0.375 86.5 0.4 89The data show that the transition temperature of pyruvate-rich xanthanis approximately 5 to 10° C. lower than a standard commercial xanthangum at equivalent salt concentrations. The temperature at which theconformational transition occurs has also been shown to be dependent onthe pyruvic acid content of the xanthan molecule and that the transitiontemperature decreases with increasing pyruvate content. These resultsare consistent with the high pyruvate content of the pyruvate-richxanthan. The salt dependence of the transition temperature is verysimilar for both samples as shown by the similar slope.Solutions of the materials tested were prepared by dissolving 0.1 weight% of the material in “fresh” (deionized) water and synthetic sea waterat 25° C. Each material dissolved easily in both distilled and saltwater.Synthetic sea water was obtained from VWR, catalog number RC8363-1. Themajor ion composition of this synthetic sea water is shown below. Othersynthetic brines for specific target oil reservoirs are formulated basedon ICP (inductively coupled plasma) analysis of the authentic brines andon the ion analysis (using ion chromatography analysis) and TDS analysisusing refractive index.

TABLE 4 Major ion concentration of synthetic sea water RC8363-1 IonIonic concentration at 34 g/kg salinity Chloride 18,740 Sodium 10,454Sulfate 2,631 Magnesium 1,256 Calcium 400 Potassium 401 Bicarbonate 194Borate 6 Strontium 7.5 Total dissolved solids 34,089.5The ratio of viscosity for each test solution was determined fromviscosity measurements taken as a function of shear rate using aBrookfield DV-II+ Pro instrument (Brookfield Engineering Laboratories,Inc., Middleboro, Mass.) using a UL adaptor with water jacketed cup andremote temperature detection probe. The instrument was controlled usingRheocal software v2.7. The shear rate was varied from 0.25 sec⁻¹ to 250sec⁻¹ at 25 and 80° C. Values of viscosity at a shear rate of 1 sec⁻¹and 10 sec⁻¹ were used in the calculation of the shear dependentviscosity ratio. This viscosity ratio was measured at the varioustemperatures to match the likely range in the reservoir temperature.The results of the viscosity testing are summarized in Table 5. In theTables, “F” stands for fresh water and “S” stands for synthetic seawater.

TABLE 5 Viscosity at 25° C. (cP) Viscosity at 80° C. (cP) ViscosityViscosity Example Water 1 sec⁻¹ 10 sec⁻¹ Ratio* 1 sec⁻¹ 10 sec⁻¹ Ratio*1 PRX-1 F 258.24 63.83 4.05 115.92 42.48 2.73 2 PRX-1 S 151.86 40.643.74 29.35 16.21 1.81 C1 X-2 F 90.97 42.77 2.13 33.75** 15.55** 2.17**C2 X-2 S 25.68 12.55 2.05 11.0 4.77 2.31 C3 X-1 F 24.94 14.16 1.76 2.201.39 1.58 C4 X-1 S 16.87 7.63 2.21 9.54 2.86 3.33 C5 SG-1 F 30.81 14.382.14 11.0 8.18 1.34 C6 SG-1 S 257.51 56.64 4.55 8.07 2.20 3.67 C7 PA-1 F240.0 65.0 3.69 87.0 26.6 3.27 C8 PA-1 S 2.20 2.42 0.91 16.14 1.69 9.55*Ratio of the viscosity measured at that temperature and at a shear rateof 1 sec⁻¹ to the viscosity measured at that temperature and at a shearrate of 10 sec⁻¹ **Measured at 85° C.The results in Table 5 show that, in general, the biopolymers testedprovided lower viscosity in salt water than in fresh water.Succinoglycan SG-1 provided excellent viscosity enhancement in saltwater at 25° C., but had at least a 25-fold drop in viscosity whentested at 80° C. Polyacrylamide PA-1 dramatically lost its viscosifyingeffect in synthetic sea water compared to fresh water. This clearlydemonstrates the detrimental effect that salt water has on theperformance of polyacrylamide.The pyruvate-rich xanthan PRX-1 provides a higher viscosity in bothfresh and salt water than the other biopolymers and had the mostconsistent performance at both temperatures tested. It also had sheardependent viscosity ratios greater than 2.0 at 25° C. and greater than1.8 at 80° C. Commercial xanthan X-1 in salt water had a good sheardependent viscosity ratio at 80° C., but at lower viscosity levels.Commercial xanthan X-2 had intermediate performance between that ofpyruvate-rich xanthan PRX-2 and commercial xanthan X-1.Samples of PRX-1, X-2 and X-1 were tested for the effect of heat agingon viscosity performance by preparing salt water solutions and holdingthem at 85° C. for a period of 20 weeks. Samples were tested for theirviscosity at intervals during that period to determine whether extendedheat treatment would reduce the observed viscosity. Reduced viscosityover time would indicate that the biopolymer was degrading. The apparentviscosity of solutions of the various soluble polymers in salt water wasdetermined from viscosity measurements taken as a function of shear rateusing the procedure described above. Values of the apparent viscosity atshear rates of 1 sec⁻¹ and 10 sec⁻¹ are shown as a function of time forthe solution held at 85° C. in Table 6.

TABLE 6 Effect of thermal degradation on apparent viscosity at shearrates of 1 sec⁻¹ and 10 sec⁻¹ in salt water PRX-2 X-2 X-1 X-1Concentration (Weight %) Thermal treatment (weeks) 0.1 0.1 0.1 0.2viscosity (cP) measured at 85° C. and shear rate of 1 sec⁻¹ 0 18 8 23 831 26 12 10 45 4 26 16 16 29 6 22 7 7 13 8 16 9 10 14 10 18 9 8 12 12 258 7 12 16 13 5 7 9 20 14 6 6 9 viscosity (cP) measured at 85° C. andshear rate of 10 sec⁻¹ 0 10 4 8 33 1 14 4 6 25 4 14 8 5 20 6 12 6 4 10 89 4 4 8 10 11 4 3 7 12 9 3 3 7 16 8 3 3 4 20 9 3 3 6In Table 6, all polymer solutions except PRX-2 (a pyruvate-rich xanthan)showed loss of apparent viscosity with time. Remarkably, PRX-2 showed noloss of apparent viscosity. This demonstrates that it is significantlymore stable to thermal degradation at 85° C. than previous xanthan gums.The data summarized in Table 6 are plotted in FIG. 3. A least squarepower law fit to the 1 sec⁻¹ data shows that the PRX-2 has the leastdecline in viscosity versus time. That isPRX-2 viscosity=24.08*time[weeks]“−0.119X-2 viscosity=11.654*time[weeks]”−0.17X-1 viscosity=19.209*time[weeks]“−0.369X-1 (0.2%) viscosity=78.591*time[weeks]”−0.759

Viscosity Build in Salt Brines Up to 265 Ppt.

Four additional solutions of synthetic salt brines were preparedrepresenting a range of salt concentrations. The first synthetic brinecontained 265 ppt TDS as summarized in Table 7.

TABLE 7 Salt Amount (g/l) NaCl 244.8 KCl 2.7 MgCl₂•6H₂O 16.6 CaCl₂•2H₂O13.3 SrCl₂•6H₂O 0.4 BaCl₂•2H₂O 0.1 NaHCO₃ 0 Na₂SO₄ 0 Total DissolvedSalt 265.6A second salt brine was obtained from an oil reservoir in Canada. Thissecond salt brine had a total TDS measured by a refractometer of 71 ppt.A low salt brine was prepared by making a 1:1 dilution of this 71 pptbrine with fresh water resulting in a brine that measured as 36 pptusing a refractometer. Another low salt brine was prepared by dilutingthe 71 ppt salt brine with fresh water until the refractometer measured10 ppt for this brine mixture.Pyruvate-rich xanthan PRX-1 at 0.1 weight % readily dissolved in allfive of these synthetic brine solutions. In contrast, xanthan X-2 didnot readily dissolve at the highest brine concentration as evidenced byits cloudy appearance.As in the previous examples, the apparent viscosity of these solutionsof PRX-1 at 0.1 weight % in the four synthetic brines was measured froma shear rate of about 1 sec⁻¹ to over 10 sec⁻¹ and at 25° C. and 80° C.Table 8 below shows the results of these measurements. It is veryremarkable that the pyruvate-rich xanthan still maintains itsviscosifying effect across this wide range of salt concentrations.

TABLE 8 Effect of a range of salt concentration in synthetic brinesViscosity at Viscosity at 25° C. (cP) 85° C. (cP) Viscosity ViscosityBrine PPT 1 sec⁻¹ 10 sec⁻¹ Ratio* 1 sec⁻¹ 10 sec⁻¹ Ratio* 10 133 35.73.72 33 15.2 2.17 36 98 25.2 3.88 17.6 10.4 1.69 71 125 35.8 3.49 24.212.0 2.02 265 60.2 22.7 2.65 12 8.1 1.48

What is claimed:
 1. A method for recovering oil from a reservoir bywaterflooding, comprising: (a) introducing an aqueous flooding fluidinto the reservoir, wherein at least one portion of said flooding fluidcomprises a xanthan gum characterized as having a pyruvic acid contentof at least 5.0 weight (w/w); (b) displacing oil in the reservoir withsaid flooding fluid into one or more production wells, whereby the oilis recoverable.
 2. The method of claim 1 wherein the pyruvic acidcontent of the xanthan gum is at least 5.3% (w/w).
 3. The method ofclaim 1 wherein the pyruvic acid to acetic acid w/w ratio of the xanthangum is about 0.9 to about 1.3.
 4. The method of claim 1 wherein theviscosity measured at 0.1% xanthan in synthetic sea water at a shearrate of 1 s⁻¹ at 25±2° C. is at least 40 cp.
 5. The method of claim 4wherein the viscosity is in the range from 40 to 130 cp.
 6. The methodof claim 1 wherein the xanthan gum is a product of a Xanthomonas strainselected from the group consisting of strains CFBP 19, CFBP 176, CFBP64, CFBP 412, ATCC 17915, ATCC 19046, or CFBP 1847, or which is aproduct of a derivative or progeny of any one of strains CFBP 19, CFBP176, CFBP 64, CFBP 412, ATCC 17915, ATCC 19046, or CFBP
 1847. 7. Themethod of claim 8 wherein the xanthan gum is a product of strain CFBP176 or of a derivative or progeny thereof.
 8. The method of claim 1wherein the xanthan gum exhibits a temperature dependent viscosity ratioat 60° C. and 20° C. of greater than 0.85.
 9. The method of claim 1,wherein said flooding fluid is recovered, and wherein at least oneportion of said the recovered flooding fluid is reinjected into thereservoir.
 10. The method of claim 9, wherein said recovered floodingfluid is supplemented with additional xanthan gum characterized bypyruvic acid content of at least 5.0 weight % (w/w).
 11. The method ofclaim 1, wherein said aqueous flooding fluid comprises sea water, brine,production water, water recovered from an underground aquifer, orsurface water from a stream, river, pond or lake.
 12. The method ofclaim 1, wherein the at least one portion of the flooding fluid exhibitsa low viscosity during injection into the reservoir and a higherviscosity when flowing through the reservoir.
 13. The method of claim12, wherein the shear dependent viscosity ratio of the at least oneportion of the flooding fluid comprising polymer is at least 1.3. 14.The method of claim 1, wherein the aqueous flooding fluid furthercomprises one or more members selected from the group consisting of1,3-propanediol; an oligomer of 1,3-propanediol; a homopolymer of1,3-propanediol; and a heteropolymer of 1,3-propanediol, wherein saidheteropolymer is synthesized using at least one C₂ through C₁₂ comonomerdiol.
 15. The method of claim 1, wherein the aqueous flooding fluidfurther comprises starch, guar gum, sodium alginate, gum arabic ormethyl cellulose.
 16. The method of claim 1, wherein the flooding fluidis disposed of at sea, in a disposal well, or in a wastewater pond. 17.An aqueous flooding fluid for enhanced oil recovery, wherein at leastone portion of said flooding fluid comprises water and 0.007 to 3 weight% a xanthan gum characterized by pyruvic acid content of at least 5.0weight % (w/w).
 18. The flooding fluid of claim 17 wherein the viscositymeasured at 0.1% xanthan in synthetic sea water at a shear rate of 1 s⁻¹at 25±2° C. is at least 40 cp.
 19. The flooding fluid of claim 18wherein the viscosity measured at 0.1% xanthan in synthetic sea water ata shear rate of 1 s⁻¹ at 25±2° C. is in the range from 40 to 130 cp. 20.The flooding fluid of claim 17 wherein the xanthan gum exhibits atemperature dependent viscosity ratio at 60° C. and 20° C. of more than0.85.
 21. The flooding fluid of claim 17 wherein the pyruvic acidcontent of the xanthan gum is at least 5.3% (w/w).
 22. The floodingfluid of claim 17 wherein the pyruvic acid to acetic acid w/w ratio ofthe xanthan gum is about 0.9 to about 1.3.
 23. The flooding fluid ofclaim 17 wherein the xanthan gum exhibits a temperature dependentviscosity ratio at 60° C. and 20° C. of more than 0.85.
 24. The floodingfluid of claim 17 wherein the shear dependent viscosity ratio of theflooding fluid is at least 1.3.
 25. A method of making an aqueousflooding fluid for use in waterflooding, comprising: (a) providing axanthan gum characterized by pyruvic acid content of at least 5.0 weight% (w/w); (b) adding the xanthan gum to at least one portion of waterused in waterflooding.